1. Field of the Invention
The present invention relates to a process for the production of catalyst for exhaust gas cleaning, which is, for example, used for exhaust gas from an automobile engine.
2. Description of the Related Art
Catalyst for exhaust gas cleaning such as ternary system catalyst, oxidation catalyst or NOx cleaning catalyst which removes NOx in an atmosphere comprising excessive oxygen is formed by supporting an active component (such as a catalytic metal) on and/or in an inorganic carrier material which is porous.
In order that the active component is supported on/in the carrier material (i.e. on an outer surface and an inner surface of the carrier material, that is, inner surfaces of pores of the carrier material), various methods are employed such as an impregnation method in which a carrier material is impregnated with a solution containing an active component, an evaporation-to-dryness method in which a slurry of an active component solution containing the carrier material in the form of powder dispersed therein is evaporated to dryness, an ion exchange method in which a cation of a metal-containing-silicate such as zeolite (a porous crystalline material having micro pores) is exchanged with an active component and so on. For example, Japanese Patent Kokai Publication No. 243545/1992 discloses a process in which a catalytic metal as an active component such as Pt and Rh is supported on a zeolite carrier by means of the ion exchange method.
However, the catalyst in the form of powder produced by the impregnation method, the evaporation-to-dryness method or the ion exchange method is not always stable in its performance of exhaust gas cleaning. Thus, there have been problems that a NOx cleaning ratio is sometimes lower than expected, that heat resistance of the catalyst is sometimes insufficient, and that the catalytic metal is fallen off the powder catalyst when the powder catalyst is wash coated on a honeycomb support member.